alahamdan - 5:35 pm on May 13, 2012 (gmt 0)
I tested few websites again.
They are some way 302 redirecting any "not available" URL to home page or another page(Sorry first I thought its a 301, seems like all using 302 after carefully checking).(some show a 404 then redirect, and some redirect directly).
So if you start guessing, trying random URL's, you wont see a 404, you will be redirected some way or another.
Some also will do no redirect, but will serve any url you ask for, like /root/whatever will show /root/ index page.
They are ranking for the home page, no one is ranking for an not-available/redirected page, those redirected pages are not indexed for sure.